"Once a user gives explicit permission to Firefox on their first installation, they will not be prompted again for subsequent releases," Mozilla said at the time in a blog post explaining the new process.

But data from Irish Web metrics company StatCounter shows that Firefox 13 was used less during its first 20 days than was Firefox 12 over a similar span. StatCounter's numbers hint that the silent updating service did not speed up the migration to Firefox 13 or result in more users running that version.

During Firefox 12's first 20 days of availability, its share of all Firefox users increased from 3% on the day of its launch to 63% nearly three weeks later. By the 10th day, 40.1% of all Firefox users were running Firefox 12. The edition cracked the 50% mark 12 days into its release life.

In every instance, Firefox 12 beat Firefox 13.

Firefox 13's share of all Firefox versions climbed from 3% on opening day to just 50.3% by day 20. On its 10th day of availability, Firefox 13 accounted for only 32.2% of all copies of Firefox -- nearly eight percentage points less than Firefox 12 at the same point during its upgrade process -- while the browser didn't break the 50% usage bar until day 19, a week slower than Firefox 12.

It's hard for even Mozilla to say why silent updating didn't do a better job moving users to Firefox 13.

"What is the cause of slow uptake rate since 13.0.1 was released unthrottled?" asked the Firefox release engineering team in a post-mortem meeting last week that focused on Firefox 13's launch.

Firefox 13.0.1, which was released June 15, was one of Mozilla's rare emergency updates. The chem spill patched bugs that contributed to a high number of crashes by Adobe's Flash Player plug-in. Last Thursday, Adobe issued its own update for Flash on Windows' Firefox.

Mozilla said that the last time it issued an emergency patch, migration to a new version was also slow.

"Similar slow uptake was seen after 10.0.1 chem spill was unthrottled on February 12," said the company, referring to a rapid-response patch that also addressed a crash bug.

Mozilla had a theory about why chem spill updates slowed down upgrading.

"After a chem spill, users still on previous major [version] must download a full update (approximately 20MB) instead of partial [update] (approximately 8MB to 10MB)," the release engineering group said.

In that scenario, come of the users who had not opted for silent updates skipped the upgrade to Firefox 13 because of its larger download size.

Other factors may have also contributed to the poor uptake of Firefox 13.

Reaction to Firefox's "new tab" page and the revamp of the browser's Home page -- both debuted in Firefox 13 -- have been largely negative, Mozilla has admitted.

During a meeting on June 13, Mozilla noted that three-fourths of all feedback on the new tabs page was negative, with just 10% approving the changes. Response to the revised Home pages, said Mozilla, was also "generally unfavorable."

It's possible that some users have switched off automatic updates to prevent Firefox from upgrading to version 13 and its new tab and Home pages.

But StatCounter's data points to the June 15 emergency update as the most likely culprit. While Firefox 12's and Firefox 13's adoption was virtually identical through the first nine days of availability, their uptake lines departed dramatically on day 10, with Firefox 13 falling increasingly behind Firefox 12's performance.

Day 10 was June 15, the date Mozilla started shipping the chem spill Firefox 13.0.1.